How to create an image with transparent background

Question

How to create an image with GDlib with a transparent background?

header('content-type: image/png');

$image = imagecreatetruecolor(900, 350);

imagealphablending($image, true);
imagesavealpha($image, true);

$text_color = imagecolorallocate($image, 0, 51, 102);
imagestring($image,2,4,4,'Test',$text_color);

imagepng($image);
imagedestroy($image);

Here the background is black

Rosana

you have to use imagefill and fill that with allocated color (imagecolorallocatealpha) that have alpha set to 0. – Nightshirt

@noice, create an answer.. it works – Rosana

@NoICE: allocating the color is not necessary, it is a true color image. – Rodas

Answer 1

You have to use imagefill() and fill that with allocated color (imagecolorallocatealpha()) that have alpha set to 0.

As @mvds said, "allocating isn't necessary", if it is a truecolor image (24 or 32bit) it is just an integer, so you can pass that integer directly to imagefill().

What PHP does in the background for truecolor images when you call imagecolorallocate() is the same thing - it just returns that computed integer.

Nightshirt

Answer 2

Add a line

imagefill($image,0,0,0x7fff0000);

somewhere before the imagestring and it will be transparent.

0x7fff0000 breaks down into:

alpha = 0x7f
red = 0xff
green = 0x00
blue = 0x00

which is fully transparent.

Rodas

Answer 3

Something like this...

$im = @imagecreatetruecolor(100, 25);
# important part one
imagesavealpha($im, true);
imagealphablending($im, false);
# important part two
$white = imagecolorallocatealpha($im, 255, 255, 255, 127);
imagefill($im, 0, 0, $white);
# do whatever you want with transparent image
$lime = imagecolorallocate($im, 204, 255, 51);
imagettftext($im, $font, 0, 0, $font - 3, $lime, "captcha.ttf", $string);
header("Content-type: image/png");
imagepng($im);
imagedestroy($im);

Bencion

This made my transparent background white instead of black :( – Muddler

i want to generate a image with dimension equal to text length how i can achieve that? without giving static parameter(100,25) to function @imagecreatetruecolor(100, 25); – Lesh

Answer 4

This should work:

$img = imagecreatetruecolor(900, 350);

$color = imagecolorallocatealpha($img, 0, 0, 0, 127); //fill transparent back
imagefill($img, 0, 0, $color);
imagesavealpha($img, true);

Mccrea

Answer 5

This should work. It has worked for me.

$thumb = imagecreatetruecolor($newwidth,$newheight);
$transparent = imagecolorallocatealpha($thumb, 0, 0, 0, 127);
imagefill($thumb, 0, 0, $transparent);
imagesavealpha($thumb, true);
imagecopyresampled($thumb, $source, 0, 0, 0, 0, $newwidth, $newheight, $width, $height);
imagepng($thumb, $output_dir);

Lucilius

Code only answers arent encouraged as they dont provide much information for future readers please provide some explanation to what you have written – Venezuela

The function imagesavealpha() saved my day. Documentation says: sets the flag to attempt to save full alpha channel information (as opposed to single-color transparency) when saving PNG images – Kurdish

Answer 6

Sometime you will not get transparent image due to problems in PNG image. The image should be in one of the following recommended formats:

PNG-8 (recommended)
Colors: 256 or less
Transparency: On/Off
GIF
Colors: 256 or less
Transparency: On/Off
JPEG
Colors: True color
Transparency: n/a

The imagecopymerge function does not properly handle the PNG-24 images; it is therefore not recommend.

If you are using Adobe Photoshop to create watermark images, it is recommended that you use "Save for Web" command with the following settings:

File Format: PNG-8, non-interlaced
Color Reduction: Selective, 256 colors
Dithering: Diffusion, 88%
Transparency: On, Matte: None
Transparency Dither: Diffusion Transparency Dither, 100%

Erdmann